1. Field of the Invention
The present invention generally relates to a flow rate sensor for measuring an intake airflow amount in, for example, an internal combustion engine, and to a method of manufacturing a flow rate detecting device used therein. More particularly, the present invention relates to a flow rate sensor, which has a heating element and which is used for measuring the flow velocity or flow rate of a fluid according to a heat transfer phenomenon where a heat is transferred to the fluid from the heating element or from a part heated by the heating element, and to a method of manufacturing a flow rate detecting device used therein.
2. Description of the Related Art
FIGS. 16 and 17 are a side sectional diagram and a plan diagram illustrating a conventional flow rate detecting device disclosed in, for instance, Japanese Patent Publication No. 5-7659 Official Gazette, respectively.
In the device shown in FIGS. 16 and 17, an insulative supporting film 2 made of silicon nitride is formed on a top surface of a plate-like substrate 1 constituted by a silicon semiconductor. A heating element 4 made of permalloy acting as a thermo-sensitive resistance material is formed on the supporting film 2. Moreover, temperature-detecting elements 5 and 6 made of permalloy acting as a thermo-sensitive resistance material are formed on the supporting film 2 in such a manner as to face each other across the heating element 4. Furthermore, an insulative protective coat 3 made of silicon nitride is formed on the supporting film 2 in such a way as to cover the heating element 4 and the temperature-detecting elements 5 and 6.
Further, opening portions 8 are formed by removing part of each of the supporting film 2 and the protective coat 3 in the vicinity of portions where the heating element 4 and the temperature-detecting elements 5 and 6 are formed. An air space 9 is formed by removing a part of the plate-like substrate 1 through the opening portions 8 by using an etching solution that does not damage silicon nitride. Thus, a bridge 13 is formed. The temperature-detecting elements 5 and 6 are arranged in a line in the direction of flow of a measurement fluid in a plane in such a manner as to face each other across the heating element 4.
Furthermore, a reference element 7 made of permalloy acting as a thermo-sensitive resistance material is provided apart and upstream from the heating element 4 in the direction of the flow 10 of the measurement fluid.
In such a conventional flow rate detecting device, a heating electric current to be supplied to the heating element 4 is controlled by a control circuit (not shown), in a manner such that the temperature of the heating element 4 is maintained at a constant temperature which is 200xc2x0 C. higher than the temperature of the plate-like substrate 1 detected by using the reference element 7.
Heat generated in the heating element 4 is transmitted to the temperature detecting elements 5 and 6 through the supporting film 2, the protective coat 3 or a thermo-sensitive resistance film. The temperature detecting elements 5 and 6 are placed symmetrically with respect to the heating element 4, as shown in FIG. 17. Thus, when there is no air flow, no resistance difference occurs between the temperature detecting elements 5 and 6. Conversely, when an air flow occurs, the upstream temperature detecting element is cooled by air, while the downstream temperature detecting device is cooled less than the upstream temperature detecting device owing to heat transmitted from the heating element 4 to the air. For instance, when an air flow moving in the direction indicated by an arrow 10, the temperature of the upstream temperature detecting element 5 is lower than that of the downstream temperature detecting element 6. The higher the flow velocity or rate of air, the difference in value of resistance between both the temperature detecting elements 5 and 6 is increased. Thus, the flow velocity or rate of air can be measured by detecting the difference in value of resistance between both the temperature detecting elements 5 and 6.
Further, when the direction of airflow is opposite to that indicated by the arrow 10, the temperature of the upstream temperature detecting element 6 is lower than that of the downstream temperature detecting element 5. Thus, it is also possible to detect the direction of airflow.
Although FIGS. 16 and 17 illustrate a conventional thermo-sensitive flow rate detecting device of the bridge type, hitherto, similarly, a thermosensitive flow rate detecting device of the diaphragm type has been widely used.
FIGS. 18 and 19 are a sectional diagram and a plan diagram illustrating a conventional thermo-sensitive flow rate detecting device of the diaphragm type, respectively.
In the device shown in FIGS. 18 and 19, each of composing elements designated by reference numerals 1 to 10 is substantially the same as the composing element, which is denoted by the same reference numeral, of the flow rate detecting device illustrated in FIGS. 16 and 17 In the conventional flow rate detecting device shown in FIGS. 18 and 19, a recess portion 12 is formed by removing part of the plate-like substrate 1 from a back surface (opposite to the top surface on which the supporting film 2 is formed) of this substrate 1 by, for example, etching. Thus, the supporting film 2 and the protective coat 3 compose a diaphragm 14 by sandwiching the heating element 4 and the temperature detecting elements 5 and 6. With such a configuration, high strength can be obtained, in comparison with the flow rate detecting device of the bridge type shown in FIGS. 16 and 17. Therefore, this conventional flow rate detecting device of the diaphragm type is suitable for use in harsh environments, for instance, an intake air flow rate sensor of an automotive engine. Incidentally, this conventional flow rate detecting device employs the principle of detecting the flow velocity or rate of air, which is similar to the principle employed in the above-mentioned conventional flow rate detecting device of the bridge type.
Meanwhile, in such a flow rate sensor having a diaphragm structure, the diaphragm 14 is in contact with the plate-like substrate 1 along the entire circumference thereof. Thus, a large part of heat generated by the heating element 4 is transmitted to the plate-like substrate 1 through the diaphragm 14. Therefore, such a flow rate sensor has a drawback in that the flow-rate detecting sensitivity thereof is lowered and that the responsivity thereof is degraded. The lowering of the sensitivity and responsivity can be prevented to some degree by increasing the size of and decreasing the thickness of the diaphragm as much as possible, whereas the flow rate sensor has another drawback in that the strength of the diaphragm 14 becomes very low. Additionally, when the amount of the heat transmitted to the plate-like substrate 1 is increased, such heat is also transmitted to the reference element 7. This results in rise of the temperature of the reference element 7. Consequently, the measurement of a fluid cannot be accurately achieved. This may adversely affect the flow-rate detecting accuracy of the sensor.
The present invention is accomplished to eliminate the above-mentioned drawbacks of the conventional flow rate sensor.
Accordingly, an object of the present invention is to provide a thermo-sensitive flow rate sensor with high detecting-sensitivity, responsivity and reliability, and which has a temperature compensating element with high measuring-fluid temperature detecting accuracy, by constructing a diaphragm by forming a cavity in a plate-like substrate by removing partially a region other than areas, in which a heating element and the temperature compensating element are formed, of the substrate, thereby increasing the heat resistance of the substrate and decreasing the heat capacity thereof.
Further, another object of the present invention is to provide a method of manufacturing such a thermo-sensitive flow rate sensor in a simple manufacturing process.
To achieve the foregoing object, according to an aspect of the present invention, there is provided a thermo-sensitive flow rate sensor having a flow rate detecting device comprising: a plate-like substrate; a heating element and a temperature compensating element constituted by thermo-sensitive resistance films and formed on a top surface of the substrate in such a way as to be spaced apart from each other; and a low heat capacity portion formed by removing partially an area, in which the heating element is formed, of the substrate from a back surface side thereof. This thermo-sensitive flow rate sensor is used for measuring the flow velocity or flow rate of a measurement fluid according to a heat transfer phenomenon where a heat is transferred from the heating element or from a portion heated by the heating element to the measurement fluid. In this thermo-sensitive flow rate sensor, a diaphragm is constructed by forming at least one cavity by removing partially a region other than the area, in which the heating element and the temperature compensating element are formed, of the substrate from the back surface side thereof.
According to another aspect of the present invention, there is provided a method of manufacturing a thermo-sensitive flow rate sensor having a flow rate detecting device comprising: a plate-like substrate; a heating element and a temperature compensating element constituted by thermo-sensitive resistance films and formed on a top surface of the substrate in such a way as to be spaced apart from each other; and a low heat capacity portion formed by removing partially an area, in which the heating element is formed, of the substrate from a back surface side thereof. This thermosensitive flow rate sensor is used for measuring the flow velocity or flow rate of a measurement fluid according to a heat transfer phenomenon where a heat is transferred from the heating element or from a portion heated by the heating element to the measurement fluid. This method has a step of constructing a diaphragm by forming at least one cavity by removing partially a region other than the area, in which the heating element and the temperature compensating element are formed, of the substrate from the back surface side thereof. In this step, a silicon substrate having a crystal orientation of (110) is provided with the plate-like substrate. An anisotropic wet etching is performed on the silicon substrate to form the at least one cavity.